The following patent publications are representative of the most relevant prior art known to the Applicant at the time of filing of the application.
______________________________________ U.S. PATS. 3,252,827 May 24, 1966 C. G. Rose et al 3,649,310 March 14, 1972 P. C. Yates 3,813,252 May 28, 1974 A. Lipp 3,875,476 April 1, 1975 W. B. Crandall et al 3,875,477 April 1, 1975 J. I. Fredriksson et al 3,890,250 June 17, 1975 D. W. Richerson 4,174,971 November 20, 1979 N. G. Schrewelius 4,205,363 May 27, 1980 C. J. Boos et al ______________________________________ FOREIGN PATENTS 1,058,673 July 17, 1979 Canada ______________________________________
The utility of ceramic or refractory compositions for electrical devices such as resistors has been known for many years. Silicon carbide heating elements have found widespread use. In more recent years, primarily as a result of the so-called energy crunch, attention has been focused on the application of ceramic resistors as igniters in, for example, gas fired furnaces and appliances like stoves and clothes driers. An example of such an igniter is taught in U.S. Pat. No. 3,875,477. The igniter is mainly composed of 95 to 99.9% by weight of silicon carbide and 0.05 to 0.50% by weight of aluminum, with optional minor quantitites of silica and iron. The terminal connecting ends, or so-called cold ends, are formed in the conventional manner by metal coating said ends by dipping in a molten metal or flame spraying. These igniters are relatively low density bodies as a result of the process used to fabricate them viz. slip casting. Therefore they are susceptible to degradation.
An earlier approach to protecting very porous (35% by volume) silicon carbide type heating elements from oxidative degradation is that taught by U.S. Pat. No. 3,252,827. This is accomplished by first forming a porous self-bonded silicon carbide rod which is then immersed in an aqueous suspension of finely divided molybdenum disilicide for a sufficient amount of time to allow the molybdenum disilicide particles to fully penetrate the silicon carbide body and coat the walls of the pores contained therein. The body is then fired in an inert atmosphere to sinter the silicide to form the final coating. Ten to twenty five percent molybdenum disilicide can be used. While this approach is effective for a relatively large body such as a heating element e.g. having a diameter of 0.5 inch (1.27 cm) or larger, they are porous and therefore relatively mechanically weak. Therefore the teachings of this patent would not produce a commercially acceptable product where the product must have a small cross section of, for example, an igniter such as that of U.S. Pat. No. 3,875,477 with a cross sectional area of from 0.012 to 0.072 in.sup.2 (0.77.times.10.sup.-3 to 0.46.times.10.sup.-2 cm.sup.2). The composition of U.S. Pat. No. 3,252,827 would result in an igniter that would simply be too fragile for practical use.
U.S. Pat. No. 4,174,971 offers a solution to the problem of relatively weak heating elements and the like. This reference impregnates a silicon carbide body with what is referred to as a molybdenum-silicon alloy which finally ends up as 25% of the structure; the impregnant is made up of about 50% by weight of silicon and 50% by weight of molybdenum disilicide.
High strength refractory resistor compositions are taught by U.S. Pat. No. 3,890,250. The product is composed of from 50 to 90% by weight of silicon nitride and 10 to 50% by weight of silicon carbide and has a modulus of rupture in excess of 100,000 psi (689 MPa) at 20.degree. C. as measured by four point loading. The electrical resistivity varies from 1 to 1.times.10.sup.7 ohm cm. These high strength characteristics are the result of hot-pressing the mixture of powders which brings about almost complete densification. However, when this material is used as an igniter the hot zone degrades rather quickly e.g. goes from a resistance of 182.4 to 247.4 ohms after only 311 hours at 1200.degree. C. and the cold ends or tabs from 40.4 ohms to 154.4 ohms.
A dense submicron grained silicon carbide-aluminum nitride body is disclosed by U.S. Pat. No. 3,649,310 which is suitable for use as a heating element, amongst other things. The composition may also contain up to 80% of boron carbide, alumina and silicon nitride. The product is made by hot-pressing a mixture of submicron sized aluminum carbide and silicon nitride at about 2000.degree. C. The two materials react with each other to form a mixture of submicron particles of aluminum nitride and silicon carbide. The resulting material has a density close or equal to theoretical density.
U.S. Pat. No. 3,875,476 specifically teaches a heat resistant ceramic electrical igniter which is composed principally (25-88%) of silicon carbide and a pyrex glass bond (5-30%); the composition may also include 1-8% ferro-silicon, 1-10% titania, 1-20% zirconia, and 5-30% silica. While not discussed in detail and not included as an example, the reference states that there are other possible mixtures of components including some containing molybdenum disilicide MoSi.sub.2. In order to keep the terminal ends cool, the igniter of this patent has a composition, based on the above compounds, which varies from one with a relative high resistance in the center or hot-zone to one with substantially lesser resistance in the terminal ends. To avoid problems resulting from significantly different coefficients of thermal expansion, the reference employs several compositions going from the hot-zone to the terminal ends so that there is a gradual compositional transition and a resulting gradual transition in the coefficient of thermal expansion of the various portions of the igniter. This eliminates premature cracking of the igniter as a result of dramatically different thermal expansion characteristics. While the reference does not limit the configuration of the igniter, i.e. apparently the igniter may take any known shape, what is shown is a U-shaped device with the ends of the legs of the U being the terminal connector ends. The shape could of course be straight or more complexly configured as shown in Canadian Patent No. 1,058,673 and U.S. Pat. No. 3,875,477.
A complex refractory composition is the subject of U.S. Pat. No. 3,813,252. The sintered refractory material is made up of 10-20% of boron nitride with 20-80% of the remainder being selected from the group consisting of aluminum nitride, aluminum boride and silicon nitride, and 20-80% of that remainder being selected from the group consisting of graphite, boron carbide, titanium carbide, zirconium carbide, chromium carbide, silicon boride, beryllium boride, magnesium boride and calcium boride. The closest this teaching comes to the present invention is a mixture of boron nitride, aluminum nitride (or silicon nitride) and silicon carbide. What is missing is the all important molybdenum disilicide.
Another igniter for stove top burners which utilize gas is that described by U.S. Pat. No. 4,205,363. The igniter is composed essentially of silicon carbide, i.e. at least 95% silicon carbide and up to 5% of a negative doping agent in the form of such elements as nitrogen, phosphorus, arsenic, antimony and bismuth. The igniter can have a ratio of room temperature resistivity to resistivity at 1200.degree. C. of less than 12 to 1 and preferably less than 9 to 1. By contrast, the present invention igniter can be formulated to have such a ratio as high as 19.8 but more importantly as low as 0.2. Heat up time, as is well known, is critical to the successful and safe use of a resistance igniter for the purpose of igniting gas. The reference discloses response times, i.e. time for the igniter to reach about 1250.degree. C. from room temperature, in 2 or 3 seconds. To accomplish this rapid response time the igniter must be made very small in cross-section, more specifically, a cross section of 0.0002 to about 0.004 square centimeters. Thus the resulting igniter is essentially a silicon carbide hair or filament which is in turn extremely fragile. The invention igniter does not possess that shortcoming.
Lastly, Canadian Patent No. 1,058,673 discloses a complexly shaped igniter element wherein the hot-zone is made up essentially of recrystallized silicon carbide and the hot-zone includes a continuous groove therein. The silicon carbide contains an electrical resistivity modifying agent such as aluminum oxide, molybdenum disilicide, magnesium fluoride, magnesium chloride or magnesium titanate or a combination of these compounds. The quantity of electrical resistivity modifier to be added, according to the patent is about 10% by weight although as much as 25% alumina in the silicon carbide is taught. Silicon nitride is also mentioned as another electrical resistivity modifying agent usable to change the resistance of the shape and to impart desirable physical properties to the igniter.
The principal differences between the present invention and the prior art are the superiority of the invention resistor, particularly when utilized as an igniter, and the novel composition thereof which is what produces the superior results.